A far as I've seen the implementations does the following (simplified):

You need to create a ENET_MAC_IF_STRUCT for the Mac interface. Here the SEND memeber is the function that will receive the PCB from upper protocol layers.

For receiving packets you need to create a PCB pool. When a a new frame is received, a PCB is allocated from this pooll (using QGET). After this a buffer for the frame is reseved and linked to the PCB. Then calling ENET_find_receiver figures aout the receiver ( upper layer protocol, i.e IP) for the frame. Finally the PCB sent to the receiver calling the SERVICE member of the ENET_ECB_STRUCT_PTR returned by ENET_find_receiver with the PCB as argument.